Dynamics of Fracture

Abstract

With the advent ofscalable parallel computers, atomistic simulations are providing immediate insights into the nature off racture dynamics by allowing us to “see” what is happening on the atomic scale. One ofour most intriguing findings is a dynamic instability ofthe crack tip in rapid brittle fracture which prevents a crack from achieving its theoretical steady-state speed equal to the Rayleigh wave speed. Also, theory suggests that crack speeds beyond the Rayleigh speed may be forbidden. However, recent experiments, for shear dominated crack growth, report evidence to the contrary. To understand this phenomenon, we have performed molecular dynamics simulations ofcrac k propagation along a weak interface joining to strong crystals. They show that a mode I tensile crack is indeed limited by the Rayleigh wave speed, consistent with the classical theories off racture. However, a mode II shear dominated crack can accelerate to the Rayleigh wave speed and then nucleate an intersonic daughter crack which quickly accelerates to the longitudinal wave speed. Furthermore, crack speeds can even surpass longitudinal wave speed in materials exhibiting elastic stifiening behavior. This phenomenon is totally contradictory to predictions ofclassical theories.